ethdev: fix xstats size query with NULL

ethdev: fix xstats size query with NULL

Normal usage of rte_eth_dev_xstats_get is to call twice. The
first time the function is called with portid, xstats = NULL
and n = 0; this returns the number of entries in the statistics
table that need to be allocated.

The problem is that the routine adds a count value to NULL (0)
and assumes that this is a valid pointer (it isn't). Device drivers
all have a check for NULL, and this no longer matches.

Fixes: d4fef8b0d5e5 ("ethdev: expose generic and driver specific stats in xstats")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Olivier Matz <olivier.matz@6wind.com>